There is some controversy on the taxonomy of this species. Some workers consider it to contain two major color morphs (dalli-type and truei-type; see Escorza Trevino and Dizon 2000), but others consider the forms to be separate subspecies: P. d. dalli and P. d. truei (Rice 1998).

Justification:
The species is widespread and abundant, with current range-wide population estimates of more than one million animals. The species was killed in high-seas driftnet fisheries operations during the 1970s and 1980s, but these fisheries have now been banned, and by-catch levels were not considered sufficiently high to cause population declines. While incidental and directed takes in Japanese coastal waters as well as incidental takes in Russian waters are ongoing (with combined removals on the order of 20,000 annually), neither threat is likely to have caused a range-wide decline sufficient to warrant listing in a category of threat.

Dall's Porpoises are found only in the northern North Pacific Ocean and adjacent seas (Bering and Okhotsk seas, and Sea of Japan). They inhabit deep waters between about 30°N and 62°N (Jefferson 1988; Houck and Jefferson 1999), but may occasionally occur as far south as about 28°N off the coast of Baja California (Mexico), during unusually cold-water periods.

The dalli-type occurs throughout of the species’ range, from the west coast of North America to Japan.

The truei-type, identified by a broad lateral white patch, inhabits the western north Pacific, and migrates between wintering grounds off the Pacific coast of northern Japan and summer breeding grounds in the central Okhotsk Sea, and constitutes one population (IWC 2002).

The map shows where the species may occur, based on oceanography. The species has not been recorded for all the states within the hypothetical range as shown on the map. States for which confirmed records of the species exist are included in the list of native range states.

Dall’s Porpoises are common in many parts of the North Pacific, and density is high in many areas of the range. The total abundance of the species is probably over 1.2 million individuals (Buckland et al. 1993). In Alaska, the abundance is estimated at 83,400 (CV=10%) (Angliss and Outlaw 2005). Along the U.S. west coast, abundance estimates have ranged from about 35,000 to 134,000, averaging 86,000 animals (CV = 45%) between 1991 and 2005 (Barlow and Forney, in press). In the western North Pacific, the truei-type population migrating between the Pacific coasts of Japan and the central Okhotsk Sea is estimated at 217,000 (CV=0.23). The dalli-type population migrating between the Sea of Japan and the southern Okhotsk Sea is estimated at 226,000 (CV=0.15), and the dalli-type population summering in the northern Okhotsk Sea at 111,000 (CV=0.29) (IWC 1998). These estimates are subject to biases due to response to survey vessels.

The International Whaling Commission currently recognizes 11 populations of this species, based on differences in genetics, pollutant loads, parasite faunas, and distribution patterns of cow/calf pairs (IWC 2002). Three of them summer in the Okhotsk Sea, two in the Bering Sea, four in the North Pacific, and two off the U.S. coast, but the wintering grounds are unknown for many of them. The populations cannot be reliably distinguished by their external appearance at sea except for the three summering in the Okhotsk Sea, which are distinguishable based on the pigmentation and location.

This species inhabits mainly offshore deep waters colder than 18°C (Miyashita and Kasuya 1988), but may also occur in narrow channels and fjords in the western North Pacific (Jefferson 1988; Rice 1998).

Sex-biased dispersal is known to occur in this species, and this may have relevance in assessing the impact of takes on regional populations (Escorza Trevino and Dizon 2000).

Dall's Porpoises are apparently opportunistic feeders, taking a wide range of surface and midwater fish and squid, especially soft-bodied species like lanternfish (myctophids) and gonatid squid. Occasional krill, decapods, and shrimps found in porpoise stomachs are not considered normal prey (Houck and Jefferson 1999, Jefferson 2002).

The largest threats to this species have been incidental takes by salmon and squid drift net fisheries and direct takes by hand harpoon fishery in Japanese coastal waters. The driftnet salmon fisheries began in 1952 and continued until a United Nation moratorium on all high-seas driftnet fisheries came into effect (Reeves et al. 2003). The large-mesh and squid driftnet fisheries operated throughout the central and western North Pacific between about 35˚N and 47˚N, increasing in effort during the 1970s and peaking during the 1980s prior to the moratorium. Bycatch estimates are only available for 1989–1990, when about 4,000 Dall's Porpoise were estimated killed per year (Hobbs and Jones 1993). During the 1970s and 1980s, the combined high-seas driftnet fisheries likely killed tens of thousands of Dall's Porpoise, but this level would not have been high enough to cause population declines (Hobbs and Jones 1993). The estimated annual take by Japanese salmon fisheries within the United States EEZ for the period 1981-1987 ranged from 741 (1987) to 4,187 (1982), with lower levels of additional takes in Bering Sea waters outside of the U.S. EEZ (IWC 1991, T. Jefferson pers. comm.).

Incidental catches on the order of thousands of porpoise per year are ongoing in several fisheries using gillnets in the Russian exclusive economic zone (Burkanov and Nikulin 2001). Small numbers of Dall's Porpoises are also taken along the US West Coast in drift net and trawl fisheries (Carretta et al. 2006).

The Japanese hand harpoon fishery for Dall’s Porpoise started in the 1910s (Ohsumi 1972, Sawadate 1983), made a great expansion around the World War II period (Wilke et al. 1953, Sawadate 1983), then remained lower at between 5,000 and 10,000 individuals until the 1970s (Kasuya 1982). Approximately 111,500 Dall’s Porpoises were removed by hunting between 1986 and 1989 from two stocks centred in the Okhotsk Sea (IWC 1991). The Japanese government began to regulate the hand-harpoon hunt in 1989, and a catch quota was introduced in 1993. The fishery currently operates with a quota of 9,000 dalli-type Sea of Japan-southern Okhotsk Sea population and 8,700 truei-type Pacific coast-central Okhotsk Sea population (IWC 2002). The current level of reported takes are about 4% of the mean estimate of the size of the populations; however, these catch statistics might not be reliable (Kasuya 2007).

Environmental contaminants are also thought to be a threat, and high levels of organochlorines may reduce testosterone levels in males and affect calf viability, thereby influencing reproduction and survival (Subramanian et al. 1987, 1988).